Papyrus

Papyrus was once the most kind hearted monster, in all the underground now he’s the most broken, forced to make his own reality to stop himself from dusting.

While sans was still trying to recover after he’s incident, it was up to papyrus to barricade up the house and gather resources, but when papyrus left the safety of his home his mind, broke he watched his friends die right in front of his eyes when papyrus, returned he broke down and after that day sans never let papyrus go through that ever again.

One day when papyrus was aloud outside, he saw toby collapsed on the floor struggling to breathe papyrus, was shocked and he brought the dog inside and nursed him back to health, after that day toby never left papyrus’s side, and now protects papyrus from any harm.

papyrus won't go anywhere without his faithful companion because he fears that he might disappear like undyne did even though sans keeps telling him that she's gone on “vacation” he really knows that something's happened to her but papyrus plays along with it for his brother.

Sans acts very careful around papyrus, because he’s scared that if he says or does the wrong thing, he might release papyrus beast form and he just doesn't have the strength to calm down, papyrus when he’s gone feral.

When frisk returned papyrus thought it was just a random monster, that needed help but after being told that this random person was frisk,he was overjoyed and sans saw a side of papyrus that he thought was long dead.